I was using synthetic oil in my new 1975 Chevy Blazer and my 1973 Chevelle SS 350 back when they were
new. I put way over 100,000 miles of the SS 350 Chevelle and the motor was like new when I sold it in 1977 and got a new Monte Carlo.
Amsoil (Superior,WI) came out in 1972 and Mobil 1 in 1973. I first used Mobil 1 then switched to Amsoil in 1975 and have used it ever since.
20W-50 Amsoil in for sale section
btw-You would be surprised at how many mechanics and engine builders know little to nothing about lubrication. I have seen them spend $1000 of dollars rebuilding an engine and recommend the cheapest quality oil for use. So take what they say with a grain of salt. I had one tell me once that oil was oil, pick whatever one you want and go with it.